Ray Matrix Approach for the Real TimeControl of SLR2000 Optical Elements
John J. Degnan
Sigma Space Corporation
14th International Workshop on Laser Ranging
San Fernando, Spain
7-11 June 2004
Basic 1D & 2D Ray Matrices*
d
1 d
0 1
Propagationover distance dx
Optic Axis
Thin Lensof FocalLength, f
1 0
-1/f 1
Optic Axis
Mirror Surfacewith Curvature R
1 0
-2/R 1
Optic Axis
Dielectric Interface
Optic Axis
n21 0
0 n1/n2n1
ax
I
dII
0
1D 2D
IIf
I1
0
-
IIR
I2
0
-
In
nI
2
10
0
a
x
10
01=I
y
x
y
x
a
a
*Valid only for a linear optical system. We need to perform a coordinatetransformation whenever the beam changes direction
Example of Coordinate SystemChange: Canted Mirror
mirror
Optical axis
Optical axisOff-axis
Ray
api+1
api’
S-vector into page
di
di+1
pi+1
pi'
( )
s
p
s
p
iii
ii
i
iiii
x
x
sdp
sdp
ddss
a
a
q
1000
0100
0010
0001
ˆˆˆ
'ˆˆ'ˆ
2sin
ˆˆ'ˆˆ
111
1
11
-
-
¥=
¥=
¥==
+++
++
Simplified SLR2000 Transceiver*
Automated Devices•Star CCD cameraperiodically updates mountmodel•3x telescope compensates forthermal drift in main telescopefocus•Beam magnifier controlslaser spot size and divergenceat exit aperture•Risley prism pair controlstransmitter point-ahead•Variable iris controls receiverfield of view (FOV)•Quadrant detector providesfine pointing corrections
*Planned modifications in red
CCDCamera
0.4 m
0.088 m5.4XBeam
Reducer
0.04 m
0.025 m
CCDSplitter
QUADMirror Transmitter
CompensatorBlock
TelescopePit Mirror
FaradayIsolator
0.080 m
0.070 m
Polarizers
Polarizer
0.085 m 0.105 m 0.350 m
0.070 m
“Zero”Wedge
Day/NightFilters
3-elementTelephoto
Lensf=0. 85 m
0.130 m
LaserTransmitter
0.213 m
Risley Prisms
Computer-controlled
Diaphragm/Spatial Filter
(Min. Range : 0.5to 6 mm diameter)
QuadrantRangingDetector
0.200 m0.051 m
3 X Telescope
f1 = -0.1 m
0.099 m
NORTH(a = 0o)
a0 = 67.4o
f2 =0.3 m
d1
d2
d3(out of page)
f1 = 0.108 m
f2 = -0.02 m
f = .025 m
d1
p1
d1
p1
d1
p1
d2
p2
Vector out of page
Vector into page
p3
s3
d2
s2'
0.250 mWorkingDistance
0.258 m
SpecialOptics2x-8xBeam
Expander
Translation Stage
Transceiver Bench Matrices
01
10 -=L
L
LL=
333.00
267.231aMTransmitter
Quadrant Detector
Star Camera0469.2
405.0559.351 L-
LL-=cM
General Form
LL
LL=
xx
xxx dc
baM1
L-L-
LL=
101.0392.0
57.2079.01bM
Coude Mount Matrix
C1
C2
C3Telescope
TurningMirror
FromTelescopePit Mirror
AzimuthAxis
ElevationAxis
Coude TrackingMount (not to scale)
s-Vector out of page
s -Vector into page
d4
p4
d5
p5
d6
p6
d5
p5'
d4
p4'
d4
d5
d6To Telescope
d3
d3
p3'
0.64 m
0.792 m
0.31 m
InstantaneousAzimuth
out of page
gg
gg
cossin
sincos
-
--=G
eaag --= 0
G
GG=
02Cd
M
a= mount azimuth anglee = mount elevation anglea0 = azimuth angle of transceiver axis at the Coude pit mirror = 67.4o (SLR2000)dC = Coude path length = 1.742 m
Telescope Assembly Matrix
1.2819 m
1.2192 m
0.4 m
Rp = 3.048 m
Rs = - .8128 m
SecondaryMirror
Primary Mirror
TelescopeTurning Mirror
Window
ft = - 0.6 m0.6214 m 0.1905 m
From CoudeTrackingMount
SLR2000TELESCOPE
(NOT TO SCALE)
0.211 m
9.97o
s-Vector out of page
s -Vector into page
d9
p9
d8
p8
d8
p8'
Elevation Axis
d7
p7d7
p7'
d9
+ Az
+ El s9
Im
IdImM
T
TT
103 = mT = telescope magnification = 10.16
dT = 5.758 m
Total SLR2000 System Matrix
( )( )
t
xx
t
xx
xTxCxtx
xTxCxtx
xx
xxxx
m
dD
m
cC
ddddbmB
cdcdamA
DC
BAMMMM
=
=
++=
++=
--
-=G
GG
GG==
gg
gg
sincos
cossin'
''
''123
Outgoing Rays Incoming Rays
gg
gg
sincos
cossin'
''
''1
-
--=G
GG-
G-G=-
T
Tx
Tx
Tx
Tx
xAC
BDM
x = a Transmitter b Quadrant Detector c Star Camera
LL
LL=
xx
xxx dc
baM1
Star Calibrations
p-axis
s-axis
Optical Bench
Looking throughrear of Star Camera
p-axis
s-axis
Looking throughtelescope window
s sc
psc
as9
Mount Elevation Axis
To Ground
ap9
CCD array
s
p
n
n
pixel
arcgg
gege
e
a
sincos
cossecsinsecsec5.0 -=
D
D
Da = star azimuth offsetDe = star elevation offsetnp = CCD pixel columnns = CCD pixel row
eaag --= 0
Quadrant Pointing Correction
p-axis
s-axis
Optical Bench
Looking throughrear of quadrant PMT
p-axis
s-axis
Looking throughtelescope window
sqd
pqd
ap9
as9
Mount Elevation Axis
To Ground
Satellite
Q1Q2
Q3 Q4
c
c
s
p
mm
arcgg
gege
e
a
sincos
cossecsinsecsec5.10 -=
D
D
Da = azimuth pointing correctionDe = elevation pointing correctionpc = horizontal centroid coordinatesc = vertical centroid coordinate
eaag --= 0
Receiver Field of View
Da = iris diameterFOV = Full Receiver Field of View
in arcsec
FOVarc
mmDa sec
125.0=
Da
TTaTaa
TT
a
T
T
TT
T
a
a
arc
mm
rad
mxxx
rad
mx
xx
aa
a
aa
sec
125.0908.25
'908.25
'387.24'039.0
'908.250
===
G-=
G-G
G-=
rr
rr
r
r
r
r
Stepper-controlled Iris
Transmitter Point-Ahead
Optical Bench
p-axis
s-axis
Looking throughtelescope window
as9
Mount Elevation Axis
To Ground
ap9
“Apparent”Position
(receiver axis)
Future Position(transmitter
axis)
ActualSatellitePosition
p-axis
s-axis
Wedge 1
Wedge 2
Looking through RisleyPrisms toward
telescope
x1x2
Dxe
a
geg
gegt
a
a
&
&
sincoscos
coscossin
-
--= rT
rp
rp ms
p
aprp =Risley prism output angleprojected into p planeasrp = Risley prism output angleprojected into s planemT = post-Risley magnification of transmitter =30.48tr = pulse roundtrip time of flight
•
•
e
a = azimuth rate
= elevation rate
eaag --= 0
Computing Risley Prism Orientations
r
rT
rp
rp ms
p
te
ta
geg
geg
xdxd
xdxda
a
&
&
sincoscos
coscossin
sinsin
coscos
2211
2211
-
--=
+
+=
d1 = half cone angle traced by wedge 1d2 = half cone angle traced by wedge 2x1 = wedge 1 angle relative to home positionx2 = wedge 2 angle relative to home position
( ) ( ) ( )( ) ( )( )[ ]( )xdddd
xgdgdtexgdgdetax
D++
D-++D-+-=
cos2
cos(cossinsincoscos
2122
21
21211
rrTm &&
( ) ( )( ) ( ) ( )( )[ ]( )xdddd
xgdgdtexgdgdetax
D++
D-+-D-+=
cos2
sinsincoscoscos.)sin(
2122
21
21211
rrTm &&
( )[ ] ( )˛˝¸
ÓÌÏ +-+
=-≡D -
21
22
21
2221
12 2
)cos(cos
ddddteeta
xxx rrTm &&
Solve above two equations for two unknown Risley orientations x1 and x2:
x2 = x1 + Dx
Simulated LAGEOS Pass
440 460 480 500 520174.5
175
175.5Differential Wedge Angle vs Time
Dxii
deg
PCA
tii
min
440 460 480 500 520180
90
0
90
180
270
360Risley Orientations vs Time
x1ii
deg
x2ii
deg
PCA
tii
mina( ) b( )
440 460 480 500 5200.001
5 .104
0
5 .104
0.001Differential Angular Rate vs Time
Time (min)
Differential Angular Rate (deg/sec)
PCA
440 460 480 500 5200.05
0.025
0
0.025
0.05Risley Angular Rates vs Time
Time (min)
Wedge Angular Rates (deg/sec)
PCA
c( ) d( )
Azimuth- Elevation Offsets & Beam Centering
40 20 0 20 4040
20
0
20
40Transmitter Point-Ahead
Azimuth Offset (arcsec)
Elevation Offset (arcsec)
4 2 0 2 44
2
0
2
4Central Ray Trajectory on Exit Window
p-axis (increasing azimuth), cm
s-axis (increasing elevation), cm
a( ) b( )
Ray Matrices and Gaussian Beams
Paraxial ray matrix theory can be applied to gaussian beampropagation if we define the following complex parameter:
( ) ( )zj
zRzq 2)(
11
pwl
-=
If propagation from a point z0 to z can be described by the ray matrix
DC
BAM =
then the gaussian beam properties at z are given by
( )
( )0
0
1
1
)(
1
zqBA
zqDC
zq +
+=
Controlling Beam Divergence
The ray matrix which takes the transmitter beam from theRisley Prism output to the far field is of the form
'1
0
''
'1
0
''
0lim
G
GG=G
GG=
•Æ
t
tt
t
tt
r
m
m
rm
m
dm
I
rIIFF
where mt is the total transmitter magnification. From our gaussian parameter, we obtain the following for the full beam divergence
( )( )
( )( )
( )( )
2
0
02
min
2
0
02
0
112
2 ˜̃¯
ˆÁÁË
Ê+=˜̃
¯
ˆÁÁË
Ê+==
zR
z
zR
z
zmr
r
tt l
pwq
lpw
wplw
q
where w(z0) and R(z0) are the beam radius and phasefront radius of curvature out of the computer-controlled telescope in the transmit path. To first order, beam divergence varies linearly withthe lens displacement from perfect focus.
Beam Divergence vs Phase FrontCurvature at Risleys
Radius of SLR2000 primary, a = 20 cmOptimum spot radius at window*,wopt = a/1.12 = 17.9 cmPost-Risley magnification, mt = 30.48Optimum beam radius at Risley,w(zo) = wopt /mt = 5.9 mmMinimum Divergence,qmin = 0.388 arcsec
0 0.1 0.2 0.3 0.4 0.50
10
20
30
40
Inverse Phase Front Curvature, m^-1
Full Beam Divergence, arcsec
q jj q min 1p w0
2⋅
lRinv
jj⋅
ÊÁÁË
ˆ̃
˜¯
2
+⋅:=
Summary• Ray matrix approach provides us with the
mathematical tools to calculate in real time:– Scale factor and angular rotation for converting star
image offsets from center in the CCD camera toazimuth and elevation biases
– Scale factor and angular rotation for convertingquadrant centroid position to satellite pointingcorrection in az-el space
– Transmitter point ahead as a function of round triptime-of-flight and the instantaneous azimuthal andelevation angular rates
– Iris diameter (spatial filter) setting for a given receiverFOV
– Transmitter beam size divergence as a function oftransmit telescope defocus